1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) projector for irradiating an LCD panel with a luminous flux emitted from a light source lamp, by using an polarizing beam-converting optical system, and projecting on a screen or the like images modulated correspondingly to video signals, and more particularly an LCD projector intended to improve the picture quality on the projection screen.
2. Description of the Prior Art
Many proposals have been made to improve the light utilizing efficiency so that LCD projectors for projecting pictures by using a luminous flux emitted from a light source lamp can accomplish projection in a larger frame in a brighter environment.
FIG. 8 illustrates the usual configuration of an LCD projector according to the prior art. This projector has a high-brightness light source lamp 11, such as a metal halide lamp or a xenon lamp. Part of the luminous flux emitted from the light source lamp 11 is reflected by a parabolic reflector 12 to give a substantially parallel luminous flux. The luminous flux directly emitted from the light source lamp 11 or that reflected by the parabolic reflector 12 passes a filter 13, which is intended to remove such harmful components of the luminous flux as heat rays or ultraviolet rays.
The parallel luminous flux, having passed the filter 13 and been cleared of harmful rays, is successively transmitted by a first polarizing plate 13, an LCD panel 15 and a second polarizing plate 16 to come incident on a projection lens 17. Here, the first polarizing plate 14 converts the linear polarized beam coming incident on it into a linear polarized beam correspondingly to video signals. The LCD panel 15 modulates this linear polarized beam correspondingly to the video signals. The second polarizing plate 16 transmits only that component of the modulated linear polarized beam which is in the direction of the axis of transmission. The projection lens 17 enlarges and projects this transmitted linear polarized beam on a screen not shown.
In an LCD projector having such a configuration, out of the luminous flux generated by a light source consisting of the light source lamp 11 and the parabolic reflector 12, only one linear polarized component transmitted by the first polarizing plate 14 is utilized as illuminating light for the LCD panel 15. Thus, the other polarized components orthogonal to this linear polarized component are lost. This results in the disadvantage that the light utilizing efficiency cannot surpass 50% as a matter of principle.
The linear polarized components not utilized as illuminating light are converted into heat by the first and second polarizing plates 14 and 16. As a result, these first and second polarizing plates 14 and 16 are changed in quality by the temperature rise and thereby deteriorated in performance. It is not desirable for this deterioration to adversely affect the quality of projected images. However, if the quantity of light emitted by the light source lamp 11 is increased to make the projected images brighter, the performance characteristics of the first and second polarizing plates 14 and 16 will deteriorate. Moreover, the LCD panel 15 would become more likely to change in performance.
FIG. 9 illustrates another LCD projector proposed to eliminate the disadvantages pointed out above. One such proposal is described in the Gazette of the Japanese Patent Laid-open No. 1991-152523. In the LCD projector shown in this diagram, part of a luminous flux emitted from a light source lamp 21 is reflected by a light source reflector 22, and the substantially parallel luminous flux thereby obtained is brought to incidence on a polarization beam-splitter prism (PBSP) 23.
This PBSP 23 consists of two prisms, the boundary face between which constitutes a polarization beam-splitting plane 24. The p-polarized component of the luminous flux coming incident on the PBSP 23 is transmitted as it is to come incident on a reflective mirror 25. The s-polarized component is reflected by the polarization beam-splitting 24 in a direction at a right angle to the p-polarized component, and comes incident on another reflective mirror 26. The p-polarized and s-polarized components, after being transmitted by their respective retardation plates 27 and 28, are synthesized by being either transmitted by a synthesizing prism 29 consisting of a right-angle prism 29 or reflected by the emitting plane of this transmitted beam, and proceeds toward an LCD panel 31. Here, the retardation plates 27 and 28 are intended to adjust the p-polarized and s-polarized components to have the same axis of polarization as the polarizing plane of the LCD panel 31.
The p-polarized and s-polarized components, after being resynthesized by the synthesizing prism 29, successively pass a condenser lens 32, a first polarizing plate 33, an LCD panel 31 and a second polarizing plate 34, and is brought to incidence on a projection lens 35. The condenser lens 32 is intended to lead the luminous flux efficiently to the projection lens 35. The first polarizing plate 33 converts the linear polarized beam coming incident on it into a linear polarized beam correspondingly to video signals. The LCD panel 31 modulates this linear polarized beam correspondingly to the video signals. The second polarizing plate 34 transmits only that component of the modulated linear polarized beam which is in the direction of the axis of transmission. The projection lens 35 enlarges and projects this transmitted linear polarized beam on a screen not shown.
This LCD projector illustrated in FIG. 9 can unify the p-polarized and s-polarized beams into which the incident luminous flux has been split by the PBSP 23 into either one of the polarized beams and illuminate the LCD panel 31 with it. Therefore, it has the advantage of achieving a higher utilizing efficiency for the light from the light source lamp 21 than that from the light source lamp 11 in the LCD projector shown in FIG. 8.
Incidentally, in the LCD projector shown in FIG. 9, if the effective display area of the LCD panel 31 is to be sufficiently illuminated, the PBSP 23, the luminous flux incident plane 29A and the emitting plane 29B of said luminous flux of the syntheiszing prism 29 will all require a size at least equivalent to the effective display area of the LCD panel 31. Both the PBSP 23 and the synthesizing prism 29 would often be made of glass. Therefore, if these elements are large, they will be inevitably heavy and make it difficult to reduce the weight of the whole projector, which would be a disadvantage.
Furthermore, in order to efficiently utilize the luminous flux over the whole effective display area of the LCD panel 31, it is desirable for the opening area of the light source reflector 22 of the light source lamp 21, like the synthesizing prism 29, to be as large as the effective display area of the LCD panel 31. If this configuration is applied to the LCD projector illustrated in FIG. 9, the luminous flux density will greatly vary between the central and peripheral parts of the projected picture, and it will become impossible to achieve even illuminance over the whole projected picture.
FIGS. 10 and 11 are intended to illustrate this disadvantage. The profile of the illuminating beam, cut normal to the optical axis, upon arrival of the luminous flux generated by the light source lamp 21 and the light source reflector 22, both shown in FIG. 9, at the LCD panel 31, also shown in FIG. 9, is represented by a circle 41 as illustrated in FIG. 10. At this time, the effective display area 42 of the rectangular LCD panel 31 is rectangular, as shown in this FIG. 10.
The illuminating fluxes from the PBSP 23 and the synthesizing prism 29, if similarly cut normal to the optical axis, will have the same cross-sectional shape and illuminance distribution. The illuminance characteristics on the straight line linking horizontal directional points in the central part of the effective display area 42 of the LCD panel 31, represented by A and B as shown in FIG. 10, are such as shown in FIGS. 11 and 12. Here, FIG. 11 illustrates the illuminance characteristic of the luminous flux having passed the PBSP 23 on the LCD panel 31, while FIG. 12 shows the illuminance characteristic of the luminous flux reflected by the PBSP 23 on the LCD panel 31.
In the case of the LCD projector illustrated in FIG. 8, the illumination of the LCD panel 31 is accomplished by a luminous flux resulting from the overlapping of luminous fluxes having the same illuminance distribution as what are shown in these FIGS. 11 and 12. Therefore, the illuminance characteristic in this case is such as shown in FIG. 13, even more ununiform in luminous flux density between the central and peripheral parts.
This is due to the following reason. The luminous flux generated by the light source lamp 21 and the light source reflector 22 has such a general distribution characteristic that its density is greater in the central part and decreases toward the periphery. This invites a similar uneven illuminance distribution on the LCD panel 31 between the central and peripheral parts. Since the illuminance distribution on the projection screen reflects that on the LCD panel 31, the projected picture will eventually have an illuminance difference between its central and peripheral parts and become uneven in brightness.
FIG. 14 illustrates a proposal made in the Japanese Patent Application No. Hei 4(1992)-33821 (Japanese Patent Laid-open No. Hei 5(1993)-241103) and U.S. Pat. No. 5,283,600 to avert this disadvantage. According to this proposal, a substantially parallel variable polarized luminous flux, provided by a light source lamp 51 and a light source reflector 52, is separated into a p-polarized beam and an s-polarized beam, which are a couple of linear polarized beams whose directions of polarization are orthogonal to each other, by the two polarization splitting planes 55 and 56 of a PBSP 54, arranged symmetrically with respect to the optical axis. The s-polarized beam after the splitting undergoes a change in optical path by a total of two matching reflective mirrors 57 and 58 to a direction similar to the path of the p-polarized beam. Here, phase difference plates 59 and 60 arranged correspondingly to these elements performs a phase conversion to change the polarizing direction of the s-polarized beam to the same direction as the p-polarized beam.
The s-polarized beam having passed these phase difference plates 59 and 60 illuminates mainly the peripheral part of an LCD panel 64 arranged between two polarizing plates 62 and 63. The p-polarized beam, which is a luminous flux transmitted by the PBSP 54, illuminates mainly the central part of the LCD panel 64. Therefore, the illumination of the LCD panel 64 is made more uniform with the result that the illuminance distribution on the projection screen, arranged behind the projection lens 35, can be expected to become more even.
FIG. 15 illustrates how the luminous flux would arrive on the LCD panel according to this proposal. A luminous flux 71 transmitted by the PBSP 54 will have a circular cross section, resulting from the addition of semicircular luminous fluxes 72 and 73 reflected by the PBSP 54.
The illuminance characteristic on the straight line linking horizontal directional points in the central part of the effective display area 74 of the LCD panel 64, represented by A and B as in FIG. 10 whereas horizontal positional points in the peripheral boundary part are represented by C and D, is such as shown in FIG. 16. Thus, the illuminance characteristics improves after superposition.
However, this proposal described in the Japanese Patent application No. 1992-33821 involves the problem that uniformity deteriorates toward the periphery even in the central part of the effective display area 74 though the illuminance distribution on the projection screen is relatively uniform as shown in FIG. 16. FIG. 17 illustrates the illuminance characteristic on the straight line linking the horizontal directional points A and B, shown in FIG. 15, revealing an unnatural illuminance characteristic with a lower level in the central part.